1. Field of the Invention
This invention deals with load handling equipment and more particularly with a system for sensing an approaching tip over condition.
2. Antecedents of the Invention
Of paramount significance in the design of load handling equipment were safety considerations including equipment stability over a range of operating conditions, both loaded and unloaded.
Although each piece of load handling equipment was rated for a maximum work load, the fact that the equipment load was within the rated weight load range did not constitute a guarantee of stability. This was because numerous additional parameters affected stability of the equipment. For example, if a load within the maximum load weight range was loaded off center, i.e. with a center of gravity displaced forwardly of a load moment axis, beyond a rated distance, or if the load center of gravity was displaced laterally from the longitudinal equipment axis, such as when a side shifter was employed, a determination that the load weight was within the specified range did not alone assure that safe equipment operation without tip over resulted. In lift trucks, additional factors such as mast tilt angle, mast load elevation, vehicle support surface slope, i.e. ramp incline, vehicle acceleration or deceleration, centrifugal force, etc. all constituted significant additional factors affecting stability.
Although systems have been devised for determining load weight, such as that disclosed in U.S. Pat. No. 5,105,896, knowledge of the load weight alone was insufficient. In U.S. Pat. No. 3,841,493, the load moment in a crane having an elongate boom was monitored by sensing the hydraulic pressure differential in a hydraulic cylinder employed to support the boom. Knowledge of the load moment alone was not sufficient to assure equipment stability, however, due to the many other variable factors which affected the dynamic interaction between load moment and equipment counterbalance moment.
It has also been proposed to detect an overload condition of a fork lift vehicle by utilizing a strain guage in a mounting bracket of a tilt cylinder, as disclosed in U.S. Pat. No. 3,993,166. The disclosed system was imprecise and did not provide an assessment of vehicle stability.
U.S. Pat. No. 5,224,815 described a load state monitoring system for a lift truck. The system employed strain gages to sense both horizontal and vertical mast bearing forces on a horizontally spaced pair of pivot bearings. The sum of sensed vertical forces was to correspond to the static load supported by the mast and the sum of the sensed horizontal forces was to correspond to the load moment.
As with the load moment monitoring system of U.S. Pat. No. 3,841,493, detection of the load moment alone was not determinative of equipment stability. Knowledge of the effect of the counterbalance moment was essential in an assessment of stability.
The prior systems did not sense the effects of the instantaneous equipment counterbalance moment which was a function of many variables, such as the weight of the lift truck operator, the employment of auxiliary extension weights on the back of the equipment to increase the load capacity, the angle of incline of the support surface, the elevation of the load, the mast tilt angle, vehicle deceleration or acceleration, centrifugal force, the employment of mast accessories, etc.
Essentially, the previous systems which attempted to guage equipment stability merely employed a deduction process for approximating stability since no assessment was made with respect to the instantaneous counterbalance moment and its effect on stability.
Further, a true stabilization sensing system must recognize when the overall center of gravity is about to be transferred to a point outside of a polygon defined by the contact points between support members, e.g. wheels, and a support surface.
Prior systems were unable to recognize a reverse tip over condition, that is, a longitudinal axis tip over in the counterbalance moment direction, which could occur with an elevated load and equipment traversing an upwardly sloped ramp, with or without the mast being tilted rearwardly.
Additionally, stabilization systems must also sense and respond to an approaching lateral tip over condition, as a result of centrifugal force, lateral ramp slope or a laterally offset load center of gravity.
Safety standards for lift trucks have been adopted by the American National Standards Institute (ANSI). The standards are entitled "Safety Standard for Low Lift and High Lift Trucks, ASME B56.1-1993 copyright 1994, The American Society of Mechanical Engineers.
Part III of ANSI B56.1, entitled "Design and Construction Standards", Section 7.6 et seq. sets forth the stability criteria for lift trucks and standards for measurement of a truck's resistance to overturning under controlled static conditions which include consideration for dynamic factors encountered during equipment operation. The testing criteria recognized factors which influenced stability including load weight, weight distribution, wheel base, wheel tread, method of suspension, truck speed, as well as tire and mast deflection under load.
Different stability tests have been established for counterbalanced lift trucks (Section 7.7), narrow aisle high lift trucks (Section 7.8), high lift order picker trucks (Section 7.9), counterbalanced front/side loader lift trucks (Section 7.10) and single sided loader lift trucks (Section 7.11).
All of the tests involved placing the equipment on a tilting platform which comprised a rigid flat surface and tilting the platform to the slope specified for each of the required tests. The truck was considered stable if it did not physically overturn when the test platform was tilted to the specified slope values.
For counterbalanced lift trucks, the platform tests for longitudinal stability included a stacking test, with the mast raised and forks carrying a test load, as well as a test simulating travelling conditions, with the test load being lowered and the mast rearwardly tilted. Lateral stability tests for counterbalanced lift trucks included a stacking test with a test load carried in the mast uppermost position and rearwardly tilted as well as a travelling test without a load.
It was evident that prior systems for detecting a potential tip over condition, which assessed stability as a function of either the load weight or the load tip over moment, merely served to deduce a possible approaching equipment tip over state, since the variable factors affecting the counterbalance moment and other equipment stability considerations were not taken into account.